Grease and method for softening grease

ABSTRACT

A grease of the invention includes: a base oil containing hydrocarbon; and diurea that is represented by a formula (1) below as a thickener and is in a range from 15 mass % to 25 mass % based on a total amount of a composition, the grease exhibiting a worked penetration in a range from 170 to 295. 
       R 1 NHCONHR 2 NHCONHR 3    (1)
 
     In the formula, R 1  and R 3  are mutually different and represent a hydrocarbon group having 6 to 18 carbon atoms; and R 2  is a divalent aromatic hydrocarbon group having 6 to 13 carbon atoms.

TECHNICAL FIELD

The present invention relates to a grease and a method for softening thegrease, more specifically, low dusting grease and a method for furthersoftening the grease.

BACKGROUND ART

A precision electronics manufacturing device such as a semiconductormanufacturing device and a liquid crystal manufacturing device requiresvery clean environments and, therefore, is set in a clean room.Moreover, a food manufacturing plant, a medicine manufacturing plant andthe like also require clean environments in order to avoid invasion offoreign substances into products. Machinery and devices used in suchenvironments are provided with various bearings, sliding portions andconnection portions. In lubricating portions of the bearings, slidingportions and connection portions, low dusting grease is used.

As the low dusting grease, a fluorine grease is exemplarily used.However, since the fluorine grease is generally expensive and lacks oflubricity, torque of the bearings filled with the fluorine grease may beincreased. Moreover, in manufacturing precision electronics, mixing of ahalogen component is unfavorable. Further, low dusting characteristicsare not sufficient. As low dusting grease containing no halogen, lithiumsoap grease and lithium complex soap grease are used. There has beenexemplarily proposed a grease containing, as the thickener, 15 to 30% ofa lithium salt of a hydroxyl-free fatty acid having 10 or more carbonatoms, relative to a whole composition, the lithium salt being blendedin a form of a fiber with 2 μm or less of each of a length and adiameter (see Patent Literature 1). Since the grease contains a metalsalt, when the grease is used in a semiconductor manufacturing deviceand a lubricating agent is splashed, production failure may be caused.Further, low dusting characteristics are also not sufficient.

Because of the above problems, a urea grease is also used. For instance,there has been disclosed a grease containing 50% or more of an ester oilas the base oil and 20% or more of an aliphatic diurea thickener andexhibiting 220 to 300 of a worked penetration (see Patent Literature 2).Since a large amount of the ester oil is contained in the grease,hydrolysis resistance may be affected to shorten a lifetime of thegrease. Further, low dusting characteristics are also not sufficient.

It is also reported that a base oil containing no ester is used in someurea greases. (see Patent Literature 3) Since such a grease has a workedpenetration in an applicable range of 190 to 230 (i.e., hard), it isdifficult to apply the grease to a usage part. Moreover, torque may beincreased and lubricity may be decreased. Further, low dustingcharacteristics are also not sufficient.

CITATION LIST Patent Literature(s)

Patent Literature 1: JP-A-2004-352953

Patent Literature 2: JP-A-2005-272764

Patent Literature 3: JP-A-11-166191

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As described above, in a typical urea grease, it is difficult to providean appropriate consistency and to reduce an amount of generated dust.

An object of the invention is to provide a grease exhibiting lowerdusting characteristics in view of the above related art. Further, whenthe grease is softened (i.e., when a consistency is increased), dustingcharacteristics are generally increased. Accordingly, an object of theinvention is also to provide a method for reducing the dustingcharacteristics while increasing the consistency.

Means for Solving the Problems

In order to solve the above problem, the invention provides a grease anda method for softening the grease as described below.

[1] A grease according to an aspect of the invention includes: a baseoil including hydrocarbon; and diurea that is represented by a formula(1) below as a thickener and is in a range from 15 mass % to 25 mass %based on a total amount of a composition, the grease exhibiting a workedpenetration in a range from 170 to 295,

R₁NHCONHR₂NHCONHR₃   (1)

in which R₁ and R₃ are mutually different and represent a hydrocarbongroup having 6 to 18 carbon atoms; and R₂ is a divalent aromatichydrocarbon group having 6 to 13 carbon atoms.

[2] In the grease according to the above aspect of the invention, aworked penetration of the grease is in a range from 230 to 265.

[3] In the grease according to the above aspect of the invention, thehydrocarbon group is a saturated hydrocarbon group.

[4] In the grease according to the above aspect of the invention, thehydrocarbon group is an alkyl group or a cycloalkyl group.

[5] In the grease according to the above aspect of the invention, thecycloalkyl group is a cyclohexyl group.

[6] In the grease according to the above aspect of the invention, R₁ andR₃ are an octyl group or a cyclohexyl group.

[7] In the grease according to the above aspect of the invention, anaverage ratio of the cycloalkyl group in the diurea is in a range from15 mol % to 85 mol % based on a total amount of the alkyl group and thecycloalkyl group as functional groups.

[8] In the grease according to the above aspect of the invention, theaverage ratio of the cycloalkyl group is in a range from 15 mol % to 50mol %.

[9] In the grease according to the above aspect of the invention, theaverage ratio of the cycloalkyl group is in a range from 75 mol % to 85mol %.

[10] In the grease according to the above aspect of the invention, acontent of the hydrocarbon group is 90 mass % or more based on a totalamount of the base oil.

[11] In the grease according to the above aspect of the invention, thehydrocarbon group is a poly-α-olefin.

[12] In the grease according to the above aspect of the invention, akinematic viscosity at 40 degrees C. of the base oil is 400 mm²/s orless.

[13] According to another aspect of the invention, a softening method ofthe grease according to the above aspect of the invention includes:stirring the grease while applying centrifugal force on the grease.

Since a predetermined amount of diurea having a specific structure isused as the thickener in the grease of the invention, an amount ofgenerated dust is extremely small. Accordingly, the grease according tothe above aspect of the invention is suitable for machinery and devicesused particularly in a clean room. Further, when the grease is softened(i.e., when a consistency is increased), the amount of the generateddust is generally increased. However, by stirring the grease accordingto the above aspect of the invention while applying centrifugal force onthe grease, the dusting characteristics of the grease can be furtherreduced although the consistency thereof is further increased.

DESCRIPTION OF EMBODIMENT(S)

A grease according to an exemplary embodiment (hereinafter, alsoreferred to as “the present grease”) is provided by containing ahydrocarbon base oil and diurea having a specific structure as athickener.

An exemplary embodiment of the invention will be described below indetail.

Base Oil

A base oil used for manufacturing the present grease containshydrocarbon as a main component. When the base oil is an ether oil(e.g., alkyl diphenyl ether) and an ester oil, dusting characteristicsof the grease are unfavorably increased. Further, when the base oilhaving such a polar group is used, hydrolysis may occur to impair afunction of the grease. In view of the above, it is preferable to avoidmixing the above oils as much as possible. The base oil preferablycontains 90 mass % or more of hydrocarbon, most preferably, consists ofhydrocarbon.

The hydrocarbon used as the base oil is preferably a poly-α-olefin interms of low dusting characteristics.

A kinematic viscosity at 40 degrees C. of the base oil is preferably 400mm²/s or less. When the kinematic viscosity at 40 degrees C. exceeds 400mm²/s, stirring resistance is increased to excessively increase torque,thereby deteriorating wear resistance. Particularly, fretting wear maybe increased. Accordingly, the kinematic viscosity at 40 degrees C. ofthe base oil is preferably 100 mm²/s or less, more preferably 50 mm²/sor less.

On the other hand, when the kinematic viscosity at 40 degrees C. of thebase oil is less than 10 mm²/s, volatility may be increased and loadresistance may be lowered.

When the kinematic viscosity at 40 degrees C. of the base oil is in arange from 10 mm²/s to 50 mm²/s, since volatility is sufficiently lowand torque is reducible, the grease is advantageously used under a lowload. The kinematic viscosity at 40 degrees C. of the base oil ismeasured in accordance with JIS K 2283.

Thickener

A thickener used for manufacturing the present grease is diurearepresented by a formula (1) below.

R₁NHCONHR₂NHCONHR₃   (1)

Herein, R₁ and R₃ independently represent a hydrocarbon group having 6to 18 carbon atoms. R₁ and R₃ in diurea of the formula (1) are mutuallydifferent. Diurea of the formula (1) is generally used as a mixture.Since R₁ and R₃ are mutually different, an average ratio of a saturatedhydrocarbon group in each of R₁ and R₃ is less than 100 mol %. When theaverage ratio of one of the above functional groups is 100 mol %, afavorable worked penetration and low dusting characteristics may not besimultaneously achieved.

Particularly, the average ratio of a cycloalkyl group in diurea ispreferably in a range from 15 mol % to 85 mol % based on a total amountof an alkyl group and the cycloalkyl group (functional groups) in termsof low dusting characteristics.

Moreover, the average ratio of the cycloalkyl group is preferably in arange from 15 mol % to 50 mol % or in a range from 75 mol % to 85 mol %.

Herein, the hydrocarbon group having 6 to 18 carbon atoms is classifiedinto a saturated one and an unsaturated one. The saturated hydrocarbongroup is defined as an alkyl group and a cycloalkyl group. Theunsaturated hydrocarbon group is defined as an alkenyl group. The alkylgroup preferably has a linear structure. The alkyl group is preferablyan octyl group in terms of low dusting characteristics. The cycloalkylgroup is preferably a cyclohexyl group in terms of balance between theworked penetration and the low dusting characteristics. The alkenylgroup is exemplified by an oleyl group.

Particularly, R₁ and R₃ are preferably an octyl group and a cyclohexylgroup.

R₂ is a divalent aromatic hydrocarbon group having 6 to 13 carbon atoms.The divalent aromatic hydrocarbon group is exemplified by4,4′-diphenylmethyl group and a toluyl group.

Diurea of the formula (1) can be easily manufactured by reacting anaromatic diisocyanate and monoamine. The aromatic diisocyanate isexemplified by diphenylmethane-4,4′ -diisocyante andtolylenediisocyanate. Monoamine is exemplified by octylamine andcyclohexylamine.

The Present Grease

The grease according to the exemplary embodiment contains the above baseoil and the above thickener. The worked penetration of the greaseaccording to the exemplary embodiment is in a range from 175 to 295,preferably from 180 to 275, more preferably from 180 to 265, furtherpreferably from 230 to 265.

Herein, a content of the thickener needs to be in a range from 15 mass %to 25 mass % based on the total amount of the composition, morepreferably from 15 mass % to 20 mass %. When the content of thethickener is small, the worked penetration of the grease becomesexcessively high, thereby increasing the amount of the generated dust.On the other hand, when the content of the thickener is excessivelylarge, the worked penetration of the grease becomes low, resulting in ahard grease. In this case, although the amount of the generated dust issmall, penetration (permeability) of the grease to a friction surfacebecomes insufficient, thereby causing a poor lubrication.

When the thickener is manufactured by mixing diisocyanate and amine atthe same equivalent and is mixed with the base oil to manufacture thegrease, the content of the thickener is equal to a content ofdiisocyanate and amine providing the thickener.

Since a predetermined amount of diurea having a specific structure isused as the thickener in the grease according to the exemplaryembodiment, the dusting characteristics of the grease are extremely low.Further, since the base oil is formed of hydrocarbon, the grease alsoexhibits an excellent hydrolysis resistance. Particularly, although theworked penetration of the grease is in an easily applicable range of 230or more to show a soft grease, the grease also exhibits low dustingcharacteristics. Moreover, since not necessarily containing halogen anda metal salt, the grease is suitable for a precision electronic device.

Softening Treatment of The Present Grease

By stirring the present grease by centrifugal force, the workedpenetration of the present grease is increased, but the dustingcharacteristics thereof are decreased. Generally, when the consistencyof the grease is increased, the dusting characteristics are increased.Accordingly, the grease having the above performance is noticeable

The centrifugal force is preferably 200 G or more, more preferably 300 Gor more.

In the present grease according to the exemplary embodiment, additivessuch as an antioxidant, rust inhibitor, solid lubricant, filler,oiliness agent, and metal deactivator may be added as needed within arange where an object of the invention is achieved.

Examples of the antioxidant include: an amine antioxidant such asalkylated diphenylamine, phenyl-α-naphthylamine andalkylated-α-naphthylamine; and a phenol antioxidant such as2,6-di-t-butyl-4-methylphenol and 4,4′-methylenebis(2,6-di-t-butylphenol). These antioxidants are used in aratio of the order of 0.05 mass % to 2 mass %.

Examples of the rust inhibitor include: sodium nitrite, petroleumsulphonate, sorbitan monooleate, fatty acid soap and an amine compound.

Examples of the solid lubricant include polyimide, PTFE, graphite, metaloxide, boron nitride, melamine cyanurate (MCA) and molybdenum disulfide.

The above various additives may be contained singly or in combination ofseveral kinds thereof.

EXAMPLES

Next, the invention will be further described in detail with Examplesand Comparatives, which by no means limit the scope of the invention.

Specifically, various greases were manufactured by a later-describedmethod and properties of the greases were evaluated by comparison. Eachof the properties of the greases was obtained in accordance with methodsdescribed below.

-   (1) Worked Penetration of Grease

A worked penetration was measured in accordance with JIS K 2220.7.

-   (2) Amount of Generated Dust (Dust Generation Test)

Using a ball screw provided in a clean room of the class 2 defined inaccordance with ISO 14644-1, a degree of dust generation from each ofthe greases was evaluated. Specifically, a 10-g grease was filledentirely over a screw surface of a ball screw (diameter: 16 mm, lead: 8mm). A 50-hour operation was carried out under the conditions of aball-nut speed being 100 mm/s and a stroke being 150 mm. Air was suckedfrom an intake port (sucking speed: 3 L/min) provided immediately nearthe screw at a middle of reciprocation. Fine particles of 0.3 μm or morewere counted by a particle counter (manufactured by RION CO., LTD.:KC-03B) and defined as a dust generation number. A total counted numberduring the test time (50 hours) was shown in piece(s) per 10L andevaluated based on the following standard.

A: 0 piece/10 L to 500 pieces/10 L

A: 501 pieces/10 L to 1500 pieces/10 L

A: 1501 pieces/10 L or more

Example 1

A grease was manufactured as follows. It should be noted that thepoly-a-olefin used as the base oil was also used in other Examples andComparatives. Properties of the poly-α-olefin were as follows.

Kinematic Viscosity at 40 Degrees C: 46 mm²/s

Kinematic Viscosity at 100 Degrees C: 7.7 mm²/s

VI:137

Density at 15 Degrees C: 0.83 g/cm³

Manufacturing of Grease

In a separate flask, 100 g of a poly-α-olefin (the base oil) and 24.8 gof diphenylmethane-4,4′-diisocyanate (MDI: COSMONATE PH manufactured byMitsui Chemicals, Inc.) were put. After a stirring fin was inserted, theobtained mixture was heated with stirring at 60 degrees C. at 250 rpmunder a nitrogen stream. After MDI was dissolved (about 15 minuteslater), a mixed amine (4.96 g of octylamine and 15.3 g ofcyclohexylamine) dissolved in 50 g of the base oil was slowly added tothe above resultant solution. The resultant solution was vigorouslystirred once so as to entirely flow, whereby the solution was uniformed.A container of the mixed amine was washed with 5.0 g of the base oil.The used base oil was added to the separate flask. The obtained solutionin the separate flask was vigorously stirred again.

Next, after the resultant solution was heated to 160 degrees C. andreached the maximum temperature, the maximum temperature was kept forone hour (while the maximum temperature was kept, the solution wasvigorously stirred about once every 15 minutes, whereby the entiresolution was uniformed. Subsequently, the resultant solution was stoppedfrom being heated and was gradually cooled. After the resultant solutionwas cooled down to the room temperature, a finish treatment wasconducted twice to the obtained product using a three-roll mill, so thatgrease was obtained.

Softening Treatment of Grease

The grease obtained by the above method was stirred under 400 G ofcentrifugal force using a rotation-revolution stirrer (MAZERUSTARmanufactured by KURABO INDUSTRIES LTD.) until the consistency of thegrease became 2.5 to 2. A composition and evaluation results of thegrease are shown in Table 1.

Example 2

A grease of Example 2 was manufactured in the same manner as in Example1 except that the amounts of the reagents to be used were changed to18.9 g of MDI, 11.3 g of octyl amine, and 5.80 g of cyclohexylamine.Then, the softening treatment was conducted to the grease of Example 2under the same conditions. A composition and evaluation results of thegrease are shown in Table 1.

Example 3

A grease of Example 3 was manufactured in the same manner as in Example1 except that the amounts of the reagents to be used were changed to27.7 g of MDI, 22.3 g of octyl amine, and 4.01 g of cyclohexylamine.Then, the softening treatment was conducted to the grease of Example 3under the same conditions. A composition and evaluation results of thegrease are shown in Table 1.

Example 4

A grease of Example 4 was manufactured in the same manner as in Example1 except that the amounts of the reagents to be used were changed to27.7 g of MDI, 22.3 g of octyl amine and 4.01 g of cyclohexylamine andthe rotation speed of the stirring fin during the manufacturing of thegrease was changed from 250 rpm to 400 rpm. However, the softeningtreatment was not applied to the obtained grease. A composition andevaluation results of the grease are shown in Table 1.

Example 5

A grease of Example 5 was manufactured in the same manner as in Example1 except that the amounts of the reagents to be used were changed to27.7 g of MDI, 22.3 g of octyl amine, and 4.01 g of cyclohexylamine.However, the softening treatment was not conducted to the obtainedgrease. A composition and evaluation results of the grease are shown inTable 1.

Comparative 1

A grease of Comparative 1 was manufactured in the same manner as inExample 1 except that the amounts of the reagents to be used werechanged to 21.3 g of MDI and 21.3 g of octyl amine and cyclohexylaminewas not used. Then, the softening treatment was conducted to the greaseof Comparative 1 under the same conditions. A composition and evaluationresults of the grease are shown in Table 1.

Comparative 2

A grease of Comparative 2 was manufactured in the same manner as inExample 1 except that the amounts of the reagents to be used werechanged to 30.5 g of MDI and 23.4 g of cyclohexylamine and octyl aminewas not used. Then, the softening treatment was conducted to the greaseof Comparative 2 under the same conditions. A composition and evaluationresults of the grease are shown in Table 1.

Comparative 3

A grease of Comparative 3 was manufactured in the same manner as inExample 1 except that the amounts of the reagents to be used werechanged to 12.3 g of MDI, 9.84 g of n-octyl amine, and 1.89 g ofcyclohexylamine. However, the softening treatment was not conducted tothe obtained grease. A composition and evaluation results of the greaseare shown in Table 1.

Comparative 4

A grease of Comparative 4 was manufactured in the same manner as inExample 1 except that the amounts of the reagents to be used werechanged to 11.6 g of MDI and 24.4 g of n-octadecyl amine andcyclohexylamine and octylamine were not used. However, the softeningtreatment was not conducted to the obtained grease. A composition andevaluation results of the grease are shown in Table 1.

Reference Example

Evaluation results of a commercially available low dusting grease(AFE-CA manufactured by THK CO., LTD) are shown in Table 1.

TABLE 1 Compar- Reference Example Example Example Example ExampleComparative Comparative Comparative ative 4 Ex. 1 2 3 4 5 1 2 3 4 5Thickener  15  13  18  18 18  14  18  12  15 — Amount (mass %) TerminalC8  20  60  80  80 80 100  0  80 — functional group CyC6  80  40  20  2020  0 100  20 — (mol %) C18 — — — — — — — — 100 Rotation speed of 250250 250 400 250  250 250 250 250 stirring fin (rpm) Aftertreatment(Softening) yes yes yes no no yes yes no no Worked penetration 259 274262 256 213  262 260 262 302 280 Consistency Evaluation A A A A A A A AC A Average particle number 380  24  58 219 77 2082  3694  754 1500  525(piece/10 L) Evaluation of dust generation A A A A A C C B C B

EVALUATION RESULTS

It is found that each of the greases of Examples 1 to 5 exhibitsextremely low dusting characteristics as compared with the greases ofComparatives 1 to 4 and Reference Example (commercially availablegrease). On the other hand, in Comparatives 1 and 3, due to the smallcontent of the thickener for manufacturing the grease, even though avalue of the worked penetration is favorable, the amount of thegenerated dust is large. In Comparative 2, since the worked penetrationexceeds the range defined by the invention, the amount of the generateddust is large. In Comparative 4, since the amount of the thickener issmall, the amount of the generated dust is slightly large although thevalue of the worked penetration is favorable.

As shown in Examples 4 to 5, even the aftertreatment (softeningtreatment) by MAZERUSTAR is not performed, the amount of the generateddust is sufficiently low.

1. A grease comprising: a base oil comprising hydrocarbon; and a diureaof formula (1) as a thickener in a range from 15 mass % to 25 mass %based on a total amount of the grease, the grease exhibiting a workedpenetration in a range from 170 to 295,R₁NHCONHR₂NHCONHR₃   (1) wherein R₁ and R₃ are mutually different andare each independently a hydrocarbon group having 6 to 18 carbon atoms;and R₂ is a divalent aromatic hydrocarbon group having 6 to 13 carbonatoms.
 2. The grease according to claim 1, wherein the workedpenetration is from 230 to
 265. 3. The grease according to claim 1,wherein the hydrocarbon group is a saturated hydrocarbon group.
 4. Thegrease according to claim 3, wherein the saturated hydrocarbon group isan alkyl group or a cycloalkyl group.
 5. The grease according to claim4, wherein the cycloalkyl group is a cyclohexyl group.
 6. The greaseaccording to claim 4, wherein R₁ and R₃ are each independently an octylgroup or a cyclohexyl group.
 7. The grease according to claim 4, whereinan average ratio of the cycloalkyl group in the diurea is from 15 mol %to 85 mol % based on a total amount of the alkyl group and thecycloalkyl group as functional groups.
 8. The grease according to claim7, wherein the average ratio of the cycloalkyl group is from 15 mol % to50 mol %.
 9. The grease according to claim 7, wherein the average ratioof the cycloalkyl group is from 75 mol % to 85 mol %.
 10. The greaseaccording to claim 1, wherein a content of the hydrocarbon is 90 mass %or more based on a total amount of the base oil.
 11. The greaseaccording to claim 1, wherein the hydrocarbon is a poly-α-olefin. 12.The grease according to claim 1, wherein a kinematic viscosity at 40degrees C. of the base oil is 400 mm²/s or less.
 13. A softening methodof the grease according to claim 1, comprising: stirring the greasewhile applying centrifugal force on the grease.